ROTATION SPLAY: If Chris Young — who allowed five earned runs through three innings last night in the Mets’ 7-5 loss to the Braves — and the rest of the Mets’ starters begin to struggle, the promise of the first half could disappear quickly, says The Post’s Joel Sherman. Photo: EPA

ATLANTA — In the first half of the season, the Mets’ rotation was akin to a good offensive line camouflaging a bad quarterback. Just in their case the bad quarterback was the majors’ worst bullpen.

The Mets were the only team that had four starters top 100 innings while pitching to an ERA of 4.10 or lower. The relief damage was overt. But imagine how much worse it would have been without consistent length and excellence from the rotation.

Right now, though, the first half feels like the good old days. Day 1 of the second half had that Waterloo-ish feel for the Mets. Dillon Gee (109 ²/₃ innings, 4.10 ERA) underwent surgery Friday to repair a damaged artery in his shoulder and is likely done for the year.

Chris Young joined the season in progress and, thus, didn’t have 100 first-half innings. But he did pitch well and that was reason for the Mets to believe the pain of Gee’s loss could be negated somewhat. But last night Young lasted just three gruesome innings, putting the Mets in a 5-0 hole — a reminder how fragile success is when you are throwing 84 mph fastballs.

Miguel Batista, the man designated to get the first crack to replace Gee, came on for long relief and walked four of the nine batters he faced. He gave way to the Mets’ newest hope of upgrading their bullpen, Josh Edgin. The young lefty began well: Four up, four down, three strikeouts. But he then suffered a familiar punch to this era — and to his ERA: He was Chipper Jonesed, as the Mets killer homered.

In all during Atlanta’s 7-5 triumph, Mets pitchers allowed 21 baserunners, 11 via walk; just the fifth time in their history they had walked that many in eight innings or fewer and the first time since April 10, 1996. This came in a game in which the power went out at Turner Field for 16 minutes in the top of the second. The Mets will hope this is not an omen for the second half. They do have their best two starters — R.A. Dickey and Johan Santana— to close this series, and the club did show superb capability to overcome hurdles in the first half. If the Mets truly are a good team, that will continue.

The Yankees, after all, lost their Nos. 1-2 starters, CC Sabathia and Andy Pettitte, on what seemed the Waterloo-ish day of June 27. But they have actually extended their AL East lead in the lefties’ absences. Of course, the Yankees have two items the Mets don’t that cover a lot of transgressions — power and a deep pen.

“When we pitch we have a chance to win,” manager Terry Collins said after a game he described as “a little frustrating.”

He recognizes the small margin of error left when it comes to a rotation that simply cannot take another hit after Gee, yet has two members (Santana and Young) not far removed from shoulder surgery. The Mets are so thin in rotation candidates, Collins continues to mull using Dickey on short rest. But he is hesitant to do so because he fears it would disrupt the preparation patterns/pride of Santana, Young and Jon Niese.

But even if Dickey were to pitch every fourth day, the Mets would still ultimately need a fifth starter. The first option is Batista, who with the aid of an off-day does not have to start until next Saturday. Nevertheless, one or two bad starts and the Mets will go to Plan B, which is Matt Harvey.

The reason he was not Plan A was the internal belief that he is not yet a finished product. Harvey has a strong fastball-curve combo, but his changeup, in particular, has not developed and his command can be erratic. The Mets want him to continue working on those items at Triple-A.

The more intriguing possibility is Zack Wheeler. The Mets’ best prospect is on the brink of a promotion from Double- to Triple-A earned with stellar first-half work. Still, the Mets prefer Wheeler finish 2012 at Triple-A, because he already is 92 ²/₃ innings into a season with a cap of around 150.

While the organization maintains long-term perspective and does not want to rush such important future pieces as Harvey or Wheeler, they also want to take a real shot at launching into the playoffs. That long shot gets way longer if the Mets suffer any more rotation setbacks.